We propose to explore the question of why in many antibody responses, only one or few B-cell clones are activated and their products expressed in preference to the antibody molecules produced by other clones. Possible explanations for the preferential expression of a limited group of antibody molecules can be based on immunologic unresponsiveness, the strength of the immunogenic stimulus, the affinities of B-cell receptors, idiotypic regulation and regulation via other cell surface proteins. These possible explanations will be tested by conducting detailed structural and physico-chemical studies on both antibody molecules and cell surface proteins.